Magnetic recording medium comprising a cured dispersion coating of sulfonated hydroxy-functional polyurethane binder and magnetizable pigment

Abstract

The present invention concerns magnetic recording media comprising fine, magnetizable pigment or particles dispersed within a binder and supported on a substrate. The invention particularly concerns the nature of the binder polymer and its preparation. The binder polymer is a curable polymer comprising a sulfonated, hydroxy-functional polyurethane polymer. A plurality of SO 3 M groups are pendant from the polyurethane polymer backbone. The SO 3 M groups have an equivalent weight of from about 2,000 to about 100,000. M is selected form the group consisting of H + , Li + , Na + , K + , NR 4 + where R can be H + or an alkyl group, and mixtures thereof. A plurality of hydrophobic polymer chain segments are incorporated into the polyurethane polymer that comprise residues of a polyol having a molecular weight of at least about 180. The hydrophobic chain segments have a carbon atom to polar functionality of at least about 60. The polyurethane polymer also comprises a plurality of cross-linkable hydroxy groups having an equivalent weight of from about 500 to about 10,000. At least a majority of the hydroxy groups are pendant from hydrophobic chain segments.

We claim: 
     
       1. A composite for magnetic recording, comprising: (a) a substrate having a front side and a backside;   (b) a cured dispersion coating on at least one side of the substrate, wherein the cured dispersion is obtained from reactants comprising a sulfonated, hydroxy-functional polyurethane polymer which comprises: (i) a plurality of SO 3  M groups pendant from the polyurethane polymer backbone wherein the polyurethane polymer has an SO 3  M equivalent weight of from about 2,000 to about 100,000, and wherein M is selected from the group consisting of H + , NR 4  + where R can be H +  or an alkyl group, Li + , Na + , K + , and mixtures thereof; and   (ii) a plurality of hydrophobic polymer chain segments incorporated into the polyurethane polymer that comprise residues of a polyol having an average molecular weight of at least about 180 and a hydroxy equivalent weight of at least about 60, wherein the hydrophobic polymer chain segments have a carbon atom to polar functionality of at least about 3:1; and   (iii) a plurality of hydroxy groups, wherein the polyurethane polymer has a hydroxy equivalent weight of from about 500 to about 10,000 and wherein at least a majority of the hydroxy groups are pendant from the hydrophobic polymer chain segments   wherein said polyurethane polymer is obtained by reacting a sulfonated polyisocyanate having at least two reactive isocyanate groups and having substantially no hydroxy functionality with a hydrophobic polyol having at least three reactive hydroxy groups, where the hydrophobic polyol is present in a sufficient amount relative to the sulfonated polyisocyanate such that there is at least about a 67% excess of reactive hydroxy groups relative to reactive isocyanate groups.     (c) a magnetizable pigment dispersed in said cured dispersion coating.   
     
     
       2. The composite according to claim 1, wherein M is selected from the group consisting of H + , Li + , Na + , K + , and mixtures thereof. 
     
     
       3. The composite according to claim 1, wherein said magnetizable pigment is oriented to a squareness ratio of at least about 0.75. 
     
     
       4. The composite according to claim 1, wherein the SO 3  M groups are aromatic SO 3  M groups. 
     
     
       5. The composite according to claim 1, wherein the SO 3  M groups are aliphatic SO 3  M groups. 
     
     
       6. The composite according to claim 1, wherein M is Na + . 
     
     
       7. The composite according to claim 1, further comprising a back-coat of the cured polymer on the backside of the substrate. 
     
     
       8. The composite according to claim 1, wherein: (a) the equivalent weight of the SO 3  M groups is from about 2,000 to 100,000; and   (b) at least about, 90%. of the hydroxy groups are pendent from the hydrophobic polymer chain segments.   
     
     
       9. The composite according to claim 1, wherein the sulfonated polyisocyanate is obtained from reactants comprising: (a) a sulfonated diol;   (b) optionally, a non-sulfonated polyol; and   (c) a polyisocyanate, wherein said polyisocyanate is present in a sufficient amount such that there is at least about a 30% excess of reactive isocyanate groups from the polyisocyanate relative to reactive hydroxy groups from the sulfonated diol and the non-sulfonated polyol.   
     
     
       10. The composite according to claim 1, wherein the hydrophobic polyol is a triol having a molecular weight of from 200 to 1000.